In series circuits of resistors, capacitors and inductances, the phenomenon of power supply, voltage and current in the same phase is called series resonance. Its characteristics are that the circuit is pure resistivity, power supply, voltage and current in the same phase, reactance X equals 0, impedance Z equals resistance R. At this time, the impedance of the circuit is the smallest and the current is the largest, which may produce high voltage many times larger than the voltage of the power supply in inductance and capacitance. So series resonance is also called voltage resonance.

Parallel resonance: In the resistor, capacitor and inductor parallel circuit, the phenomenon that the voltage at the end of the circuit and the total current are in the same phase is called parallel resonance. Its characteristics are: parallel resonance is a complete compensation, the power supply does not need to provide reactive power, only provides the active power required by the resistor. When resonating, the total current of the circuit is the smallest, while that of the branch circuit is the smallest. Current is often greater than the total current in the circuit, so parallel resonance is also called current resonance.

The characteristics and differences of series resonance and parallel resonance:

1. From the load resonance mode, it can be divided into two types: parallel inverters and series inverters. The main technical characteristics and differences of series inverters and parallel inverters are listed below.

The difference between series inverters and parallel inverters is due to their different oscillating circuits. The former is in series with L, R and C, and the latter is in parallel with L, R and C.

(1) Load circuit of series inverters presents low impedance to power supply, which requires voltage supply. Therefore, large filter capacitors must be connected in parallel at the end of rectified and filtered DC power supply. When the inverters fail, the surge current is large and the protection is difficult.

The load circuit of the parallel inverters presents high impedance to the power supply, which requires the power supply from the current source, and the large reactor should be connected in series at the end of the DC power supply. But when the inverters fail, because the current is limited by large reactance, the impact is small and easy to protect.

(2) The input voltage of the series inverters is constant, the output voltage is rectangular wave, and the output current is nearly sinusoidal wave. The commutation takes place after the current on the thyristor passes zero, so the current is always ahead of the voltage by an angle of phi.

The input current of the parallel inverters is constant, the output voltage is nearly sinusoidal, and the output current is rectangular. The commutation is carried out before the voltage on the resonant capacitor passes zero, and the load current is always ahead of the voltage-angle. That is to say, both of them work in the capacitive load state.

(3) Series inverters are supplied by constant voltage source. In order to avoid the short-circuit of power supply caused by the simultaneous turn-on of upper and lower bridge arms thyristors of inverters, it is necessary to ensure that the power is turned off first and then turned on when commutating. That is to say, all thyristors (other power electronic devices) should be turned off for a period of time (t). Stray inductance at this time, i.e. induced potential generated from the DC terminal to the lead inductance of the device, may damage the device, so it is necessary to select the appropriate device surge voltage absorption circuit. In addition, in order to ensure continuous load current and protect thyristor from high voltage on converter capacitor during thyristor switching off, fast diodes must be connected in reverse parallel at both ends of thyristor.

Parallel inverters are supplied by a constant current source. In order to avoid large induced potential on the filter reactance Ld, the current must be continuous. That is to say, it is necessary to ensure that the upper and lower arm thyristors of the inverters are switched on and off before commutation, that is to say, all thyristors are on during commutation (tgam). At this time, although the inverter bridge arm is directly connected, because Ld is large enough, it will not cause short circuit of DC power supply, but the long commutation time will reduce the efficiency of the system, so it is necessary to shorten t gamma, that is, to reduce the Lk value.

(4) The working frequency of series inverters must be lower than the inherent oscillation frequency of the load circuit, that is to say, the proper T-Time should be ensured, otherwise the commutation failure will be caused by the direct connection of the upper and lower arms of the inverters.

The operating frequency of parallel inverters must be slightly higher than the inherent oscillation frequency of the load circuit to ensure that there is an appropriate reverse voltage time t, otherwise it will lead to the failure of thyristor-to-thyristor switching; however, if it is too high, the reverse voltage of thyristor during switching will be too high, which is not allowed.

(5) There are two ways to regulate the power of series inverters: changing the voltage Ud of DC power supply or changing the trigger frequency of thyristor, that is, changing the load power factor cos_.

In general, the power regulation mode of parallel inverters can only change the DC supply voltage Ud. Although changing cos_can also increase the output voltage and power of the inverters, the allowable adjustment range is small.

(6) When series inverters commute, the thyristor is turned off naturally, and the current of the thyristor is gradually reduced to zero before switching off, so the switching off time is short and the loss is small. During commutation, the time of thyristor backpressure (t+t gamma) is longer when the thyristor is switched off.

When shunt inverters are converted, the thyristor is forced to turn off in full current operation. When the current is forced to drop to zero, a period of back voltage is needed, so the turn-off time is longer. In contrast, series inverters are more suitable for use in induction heating devices with higher operating frequency.

(7) The thyristor of series inverters has a lower voltage to withstand. When using 380V power grid to supply power, 1200V thyristor will do, but all the current of the load circuit, including active and reactive components, needs to flow through the thyristor. If the thyristor loses the pulse, the oscillation will stop and the inverters will not be subverted.

The voltage of thyristor of parallel inverters increases rapidly with the increase of power factor angle. But the load itself constitutes an oscillating current loop, only the active current flows through the inverted thyristor, and when the inverted thyristor occasionally loses the trigger pulse, it can still maintain the oscillation and work stably.

(8) Series inverters can work either by self-excitation or by other excitation. The output power can be adjusted only by changing the trigger frequency of the inverters, while the parallel inverters can only work in the self-excited state.

(9) In series inverters, the asymmetric trigger pulses of thyristors will not affect the normal operation by introducing DC component current, while in parallel inverters, the asymmetric trigger pulses of inverted thyristors will cause faults by introducing DC component current.

(10) Series inverters are easy to start and suitable for frequent start-up. Parallel inverters need additional starting circuit, which makes starting more difficult.

(11) Because thyristors in series inverters withstand rectangular wave voltage, du/dt value is larger and absorption circuit plays a key role, while di/dt requirement for thyristors is lower.

In parallel inverters, the current flowing through the thyristor of the inverter is rectangular wave, which requires large di/dt, but less du/dt.

(12) When the distance between induction heating coil of series inverters and inverters (including slot capacitors) is long, the influence on output power is small. If coaxial cables or future loops are laid as close as possible (better twisted together), there will be little impact. For parallel inverters, the induction heating coil should be as close as possible to the power supply (especially the groove capacitor), otherwise the power output and efficiency will be greatly reduced.

(13) The voltage on the induction coil and the voltage on the slot capacitor of the series inverters are Q times of the output voltage of the inverters, and the current flowing through the induction coil is equal to the output current of the inverters.

The voltage on the induction coil and the slot capacitor of the parallel inverters is equal to the output voltage of the inverters, and the current flowing through them is Q times of the output current of the inverters.

In summary, parallel inverters and series inverters (commonly known as parallel or series variable frequency power supply) have their own technical characteristics and application fields. From the point of view of industrial heating application, parallel inverters are widely used in various fields such as smelting, thermal insulation, diathermy, induction heating and heat treatment, and their power can range from several kilowatts to tens of thousands of kilowatts. Series inverters are widely used in smelting-heat preservation of one-tow-two furnace groups and induction heating occasions with high Q value and high frequency. Their power can range from several kilowatts to several kilowatts. At present, more than 90% of the variable-frequency power supply used in our country's industry belongs to parallel variable-frequency power supply.